Aluminum-copper metallurgy has been used in the prior art to avoid the electromigration problem, as is discussed, for example, by Hall, et al., in U.S. Pat. No. 3,743,894.
In the fabrication of semiconductor device, a contact metal layer of aluminum is generally used to make ohmic contact to the device. When the device is operated under high current and high temperature conditions, the aluminum contact metal is transported by the current flowing therethrough causing the metal to build up in some areas and to form voids in others. The voids can become large enough to sufficiently increase the resistance of the metal contact in the area where the voids occur to allow resistive heating to cause the contact metal to melt, thereby causing premature failure of the device.
Hall, et al., teaches that to avoid the electromigration problem, aluminum contact metallization is codeposited with a small percentage of copper on the order of 1 to 10 percent by weight. Forming a fine grain structure of CuA1.sub.2 grains having a diameter of less than 1,000 Angstroms interspersed between aluminum grains at the grain boundaries and triple points thereof.
Silicon has been alloyed with the aluminum-copper in the prior art to prevent the aluminum-copper from penetrating into the silicon substrate as is discussed, for example, by Kuiper in U.S. Pat. No. 3,382,568.
In the fabrication of silicon devices with aluminum lands, some surface passivations of silicon planar devices, e.g., glass coating, require heat exposure of the device to a temperature just below the silicon-aluminum eutectic temperature. Under some conditions, silicon from the wafer will be dissolved in the aluminum at a temperature as much as 15 degrees below the eutectic temperature, thereby resulting in higher land resistances and an unreliable device.
One hypothesis is that a stress mechanism between SiO.sub.2 and SI plays a part in this effect. Aluminum and silicon in initmate contact from a eutectic, a liquid alloy, at approximately 577.degree. C. Therefore, glassing is restricted to temperatures below 577.degree. C. When a silicon device has lands running from a contact hole in the oxide to a distant point on the oxide over the oxide layer and the device is glassed at 570.degree. C., problems arise at the stepdown where the aluminum contact stripe traverses from the oxide to the silicon. The problems are phenomena such as "necking down" or breaking of the stripe and deep vertical or lateral penetrations of the silicon by the aluminum. The electrical consequences of such behavior are, in the former instance, high resistance points which burn out and open under electrical load, or in the later case, short circuiting of the junction. The foregoing shortcomings are overcome by Kuiper through evaporating a small amount of silicon with the aluminum during the evaporation step for the formation of conductors, contacts and lands on the silicon device. It is required that the silicon be evaporated quite close to the plane of aluminum silicon contact. The small amounts of silicon thus mixed with the aluminum prevent subsequent diffusion of further amounts of silicon into the aluminum lands, lines and stepdown portions therebetween.
In the existing process for etching the aluminum-copper-silicon conductor structures, a layer of photoresist having the desired pattern is formed on the surface of the aluminum, copper, silicon composite. Thus, a phosphoric acid-nitric acid/hydrofluoric acid solution is employed at room temperature with the objective of removing the silicon layer, forming the desired pattern therein. This process step often results in leaving a silicon residue on a surface of the conductor layer because of the lack of uniformity in thickness of the silicon layer and the poor silicon etching characteristic of the etchant. Etching for extended periods with the silicon etchant results in under cutting the remaining silicon beneath the photoresist layer. The second step in the existing process employs a standard aluminum-copper etch solution of phosphoric acid and nitric acid. When the silicon layer has not been uniformly removed from the surface of the conductor layer by the previous step, aluminum-copper is etched out from beneath the unetched silicon leaving silicon projections and islands. Even when the silicon layer is uniformly removed, the aluminum-copper etchant undercuts beneath the silicon layer remaining beneath the photoresist, forming undesirable ledges. This can result in shorted conductors in the resulting product.
In the prior art, often several ultrasonic cleaning steps are required using a suitable solvent, in the attempt to remove the residue and ledges from the silicon layer. But this cleaning operation yeilds unrelaible results and indeed frequently induces stress cracking in the device structure. After this step, the silicon is sintered into the aluminum-copper, as described above.